1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device which includes a means for overcoming problems attributed to polarization sunglasses.
2. Description of the Related Art
A TFT-type liquid crystal display panel which uses a thin film transistor as an active element can display a high-definition image and hence, such a liquid crystal display panel has been used as a display device of a television receiver set, a personal computer display or the like. Particularly, a miniaturized TFT-type liquid crystal display device has been popularly used as a display part of a mobile phone.
On the other hand, as such a TFT-type liquid crystal display panel, a vertical-electric-field-type (for example, TN-type, ECB-type, VA-type or the like) liquid crystal display panel, and a lateral-electric-field-type (also referred to as IPS-method) liquid crystal display panel have been known. It is also known that the IPS-method liquid crystal display panel can acquire a broad viewing angle.
In general, the IPS-method liquid crystal display panel is constituted of a first substrate (hereinafter also referred to as a TFT substrate), a second substrate (hereinafter also referred to as a counter substrate), and liquid crystal sandwiched between the first substrate and the second substrate. The first substrate includes a first alignment film arranged on the liquid-crystal-side surface thereof and a first polarizer arranged on the surface thereof opposite to the liquid crystal, while the second substrate includes a second alignment film arranged on a liquid-crystal-side surface thereof and a second polarizer arranged on a surface thereof on a side opposite to the liquid crystal.
Further, on the liquid crystal display panel, within a region surrounded by two neighboring scanning lines (also referred to as gate lines) and two neighboring video lines (also referred to as drain lines), a thin film transistor which is turned on in response to a scanning signal from a scanning line and a pixel electrode to which a video signal from a video line is supplied via the thin film transistor are formed thus constituting a so-called sub pixel.
Conventionally, on the IPS-method liquid crystal display panel, video lines (DL) are arranged so as to be parallel to either one of long sides or short sides of a rectangular display region, and scanning lines (GL) are arranged orthogonal to the video lines (DL).
Further, an alignment axis (or a rubbing direction) of the first alignment film and an alignment axis of the second alignment film are set to the same direction, and linear portions of a pixel electrode (PX) are formed such that a narrow angle out of intersecting angles between the alignment axis of the first alignment film (or the alignment axis of the second alignment film) and an extension direction of the linear portions of the pixel electrode (PX) assumes a predetermined angle (hereinafter referred to as a pre-twist angle, being set to a value ranging from approximately −20° to 20°).
Further, an absorption axis of the first polarizer and an absorption axis of the second polarizer are set to be orthogonal to each other and, at the same time, either one of the absorption axis of the first polarizer and the absorption axis of the second polarizer is set to agree with the alignment axis of the first alignment film and the alignment axis of the second alignment film.
As described previously, it has been known that the IPS-method liquid crystal display panel can acquire a broad viewing angle. Recently, as a technique which further increases a viewing angle of the IPS-method liquid crystal display panel, there has been known a liquid crystal display device of a type which divides each sub pixel into two regions, and imparts electric fields of different directions to liquid crystal in respective regions (also referred to as a multi-domain-type liquid crystal display device). On the other hand, a liquid crystal display device of a type which imparts an electric field only in one direction without dividing each sub pixel into two regions is referred to as a single-domain-type liquid crystal display device. The multi-domain-type liquid crystal display device exhibits excellent viewing angle characteristic compared to the single-domain-type liquid crystal display device.
FIG. 11 is a view for explaining one example of the electrode structure of a conventional multi-domain-type liquid crystal display panel adopting an IPS method.
In the electrode structure shown in FIG. 11, a counter electrode not shown in the drawing is formed in a planar shape, and a pixel electrode (PX) is formed of an electrode having a plurality of slits (SLT). Portions which are divided by the slits (SLT) form linear portions (comb-teeth electrodes) (KSB) of the pixel electrode (PX). The pixel electrode (PX) and the counter electrode are formed of a transparent conductive film made of ITO (Indium Tin Oxide) or the like, for example. In FIG. 11, an extension direction of the linear portions (KSB) of the pixel electrode (PX) extends in a direction of short sides out of long sides and short sides of each sub pixel.
Further, the pixel electrode (PX) having the linear portions and the planar counter electrode are stacked to each other with an interlayer insulation film sandwiched therebetween. By generating arcuate lines of electric force generated between the pixel electrodes (PX) and the counter electrode such that the lines of electric force are distributed in the liquid crystal layer in a penetrating manner thus changing the alignment of the liquid crystal display layer (LC).
In FIG. 11, the initial alignment direction (HLC) of the liquid crystal is set parallel to the extension direction of the scanning lines (GL). Further, a region of each sub pixel is divided in two (two regions ARA, ARB in FIG. 11), and the slits (SLT) formed in the divided region ARA and the slits (SLT) formed in the divided region ARB are formed in line symmetry with respect to the initial alignment direction (HLC) of the liquid crystal.
Due to such constitution, by setting a rotational direction of liquid crystal molecules in the region ARA and a rotational direction of liquid crystal molecules in the region ARB opposite to each other with respect to a direction of the electric field applied between the pixel electrode (PX) and the counter electrode (CT), a wide viewing angle can be realized.
FIG. 12 is a plan view for explaining another example of the electrode structure of a conventional liquid crystal display panel of a multi-domain-type adopting an IPS method.
In the electrode structure shown in FIG. 12, the extension direction of the linear portions (KSB) of the pixel electrode (PX) extends in the direction of long sides out of the long sides and the short sides of each sub pixel. Further, an initial alignment direction (HLC) of liquid crystal is arranged parallel to the extension direction along which the video line (DL) extends as a whole (although the video line (DL) extends while being bent, the extension direction along which the video line (DL) extends as a whole, in this specification, does not imply the extension direction of a local portion of the video line (DL) but implies the direction along which the video line (DL) extends as a whole). Further, a region of each sub pixel is divided into two regions (the region ARA and the region ARB in FIG. 12) using a center line Lcen as a boundary, wherein slits (SLT) make an inclination angle with respect to the center line Lcen different from each other between the divided region ARA and the divided region ARB such that the slits (SLT) face each other in an opposed manner with the center line Lcen sandwiched therebetween.
Due to such constitution with respect to the direction of an electric field applied between the pixel electrode PX and the counter electrode CT, a rotational direction of liquid crystal molecules in the region ARA and a rotational direction of the liquid crystal molecules in the region ARB are set opposite to each other thus realizing a broad viewing angle.
In both cases shown in FIG. 11 and FIG. 12, the pixel electrodes (PX) and the counter electrode are formed together on the same substrate, and electric fields are generated due to differences in potential between the pixel electrodes (PX) and the counter electrode thus driving liquid crystal.
In FIG. 11 and FIG. 12, symbol a-Si indicates a semiconductor layer, symbol CH indicates a contact hole for connecting a conductive layer (SD) which functions as a source electrode and a pixel electrode (PX), and symbol CHK indicates an opening portion formed in the counter electrode. Although each slit (SLT) shown in FIG. 11 and FIG. 12 has both ends thereof closed, one end of the slit (SLT) may be opened.